This invention relates to a semiconductor photonic integrated circuit and a manufacturing method thereof. More particularly, the invention relates to a semiconductor photonic integrated circuit which, formed on the same substrate, comprises semiconductor photonic devices of different functions, and to a manufacturing method of the semiconductor photonic integrated circuit involving a selective-area growth technique using an insulating film patterning mask.
There exists what is known as the selective-area growth technique that utilizes insulating film patterning masks in integrally fabricating, on the same semiconductor substrate, such semiconductor photonic devices as the semiconductor laser, optical modulator, optical switch, photo-detector and optical amplifier of different functions. The selective-area growth technique involves primarily forming insulating film patterning masks over the semiconductor substrate so as to permit vapor phase growth of semiconductor crystals in unmasked, i.e., exposed areas of the substrate. During manufacture of target semiconductor photonic devices, the width of the insulating film mask and that of the exposed area over the semiconductor substrate are varied in the light transmission direction of these devices, and vapor phase growth of an alloy semiconductor is effected. This causes alloy semiconductor layers of different growth layer compositions and of different layer thicknesses to be formed automatically in the same process and in accordance with the width of the insulating film mask and that of the exposed area. This is because the density gradient in vapor phase of various materials that contain the atoms constituting alloy semiconductor crystals, and the effective surface migration length involved, vary from material to material. The selective-area technique process thus allows the semiconductor laser, optical modulator and other semiconductor photonic devices of different functions to be formed on the same semiconductor substrate in the same process. The photonic devices so formed have good optical coupling therebetween.
Literature on the manufacture of semiconductor photonic integrated circuits based on the selective-area growth technique includes illustratively the 1991 and 1992 transactions of the Institute of Electronics, Information and Communication Engineers of Japan (C-131 and C-178 respectively from the general meetings of autumn 1991 and spring 1992), and Japanese Patent Application No. Hei 3-180746.
It has been found that, for conventional manufacture of a plurality of growth layers of different thicknesses and different compositions, it has been found that the more the periods of quantum well active layers (i.e., the larger the number of repetitions of quantum wells), the less pronounced the quantum effect due to the nonuniformity in growth layer structure and composition. This has led to degraded characteristics of the semiconductor photonic integrated devices eventually manufactured. Another conventional finding is that, when the growth layers formed by the selective-area growth process are formed into mesa-stripes by etching and then into current block layers and the like by means of buried growth, the fabrication yield tends to drop. This is because the process of photolithography requires high levels of alignment precision so that the irregular layer thicknesses and compositions of the growth layers are taken into account.
What follows is a more detailed description of the foregoing aspects of the prior art with reference to the accompanying drawings. In FIG. 1A, an insulating film mask 2 whose width varies in the optical axis direction is formed on a semiconductor substrate 1 to effect partially vapor phase growth of an alloy semiconductor. Here, the density gradient in vapor phase of various materials that contain the atoms constituting alloy semiconductor crystals, and the effective surface migration length involved, vary from one material to another. This causes semiconductor layers 3, 4 and 5 of different compositions and thicknesses to be formed according to mask width. As shown in FIG. 1D, a cross-sectional view taken on line X-X' in FIG. 1A, areas a-1 and a-2 have a very smooth transition area therebetween because their crystals were made to grow in the same fabrication process. The structure provides high levels of optical coupling efficiency between semiconductor photonic devices of different functions formed in different areas of the substrate.
In growing the quantum well active layer 4, suppose that an open space width between masks is narrowed (illustratively to about 2 .mu.m). In that case, as depicted in FIG. 1B, even if the growth time of quantum well layers 6 (InGaAs) and that of barrier layers (InGaAsP) 7 within the quantum well active layer 4 are made constant, the layers 6 and 7 tend to vary in terms of film thickness and composition in the direction of their growth.
Suppose then that the open space width between the masks is widened (illustratively to about 80 .mu.m). In that case, as shown in FIG. 1C, the film thickness and the composition of the layers 6 and 7 are non-uniform in the direction of planes perpendicular to the optical axis inside the alloy semiconductor layers. The closer the layer to the mask, the greater the film thickness thereof; the closer the layer to the mask, the higher the In content thereof. Larger numbers of periods of quantum wells thus entail growing degrees of nonuniformity in the layer structure and composition. As a result, the quantum size effect involved is lessened, and the characteristics of the semiconductor photonic integrated circuit devices are reduced. As shown in FIG. 1C, the distribution of emission wavelengths in the quantum well active layer is arcuate in the direction of planes perpendicular to the optical axis. The closer the layer to the masks, the farther the wavelength is red-shifted. Compared with the case where the crystals are grown on an unmasked substrate, the full width at half Maimum (FWHM) in luminescence from quantum wells is increased and the device performance is lowered. Furthermore, when the growth layer formed as described is etched into mesa-stripes about 1 .mu.m wide, a slight misalignment that may occur varies the emission wavelength and degrades the fabrication yield.